CHEM 1105 S10 January 17, 2014
Chapter 2:
Sections 2.7-2.8: Atomic mass and molar mass
Atomic mass
Mole concept
Molar mass of elements
Molar mass of compounds (preview)
If time:
Begin Chapter 3: Chemical compounds
- Types of compounds and naming
Atomic mass
Non-whole number from periodic table
Don’t confuse with mass number (a whole
number)
Atomic mass of an atom of carbon-12 = exactly
12 amu (“atomic mass units”)
Each proton and neutron is approximately 1
amu, but not exactly
Atomic mass = (fraction of isotope 1) x (mass of isotope 1) +
(fraction of isotope 2) x (mass of isotope 2) +
(fraction of isotope 3) x (mass of isotope 3) +
...
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Example:
Magnesium has three naturally occurring isotopes with masses
of 23.99 amu, 24.99 amu, and 25.98 amu and natural
abundances of 78.99%, 10.00%, and 11.01%, respectively.
Calculate the atomic mass of magnesium.
Example:
Gallium has two naturally occurring isotopes: Ga-69 with a
mass of 68.9256 amu and a natural abundance of 60.11%, and
Ga-71. Use the atomic mass of gallium listed in the periodic
table to find the mass of Ga-71.
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What is a mole?
Mole is the basic unit of measurement of a
chemical substance, usually based on it’s
chemical formula
The unit MOLE is associated with 6.02 x 1023
“units” (atoms, molecules, electrons, ions,
etc.)
6.02 x 1023 is known as Avogadro’s number.
1 mole = 6.02 x 1023 units
1 mole of an element = the element’s
atomic mass in grams
1 mole Cu = 63.55 g Cu (from Periodic table)
1 mole Cu = 6.02 × 1023 Cu atoms
Mole & Mass Relationship
An element’s molar mass in grams per mole is numerically equal
to the element’s atomic mass in atomic mass units.
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Mass-to-Mole Conversions
How many Mg atoms are in 0.200 g?
Molar mass of a compound
Molar mass of a compound = sum of molar masses of
elements.
Molar mass of CO2 = 12.01 + 2(16.00) = 44.01 g/mol
Example:
How many grams of CO2 are in 6.75 x 1030 molecules of
CO2?
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Elements and Compounds Groupings
• Atomic elements :
– elements whose particles are single atoms
• Molecular elements:
– elements whose particles are multi-atom molecules
• Molecular compounds:
– compounds whose particles are molecules made of
only nonmetals
• Ionic compounds:
– compounds whose particles are cations and anions
Elements: Names and Symbols you should know
http://jenniferwolfchemistry.files.wordpress.com/
2012/10/elements.pdf
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Compounds and Molecules
• COMPOUNDS are a combination of two or more elements in
definite ratios by mass.
• The character of each element is lost when forming a
compound.
– Chemical compounds are composed of two or more
elements.
• Same or different
• Composition of molecules is given by a MOLECULAR
FORMULA.
– Example: Water’s molecular formula is H2O.
• A MOLECULE is the smallest unit of a compound that retains
the characteristics of the compound.
Molecular View of
Elements and Compounds
12
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How are the following classified?
Barium, Ba
atomic element
Iron (III) chloride, FeCl3
ionic compound
Bromine, Br2
molecular element
Ethanol, C2H6O
molecular compound
Nitrogen monoxide, NO
molecular compound
Cobalt, Co
atomic element
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